1. Field of the Invention
The present invention relates to a radiation image detector that records a radiation image by being irradiated by radiation bearing a radiation image, from which signals corresponding to the radiation image are read out by being scanned with readout light, and from which residual charges are erased by being irradiated with erasing light.
2. Description of the Related Art
Various types of radiation image detectors that generate charges by being irradiated with radiation which has passed through subjects and record radiation images of the subjects by accumulating the charges have been proposed and are in actual use in the field of medicine and the like.
For example, U.S. Pat. Nos. 6,995,375 and 7,034,312 discloses a radiation image detector constituted by: an upper planar electrode, which is formed on one side of a radiation sensitive semiconductor material; and a plurality of divided electrodes, which are formed on the other side of the semiconductor material. In this radiation image detector, radiation is irradiated onto the semiconductor material in a state in which a biasing voltage is being applied between the upper planar electrode and the divided electrodes. The irradiation of the radiation causes electric charges to be generated within the semiconductor material, and the generated charges are read out as electrical signals by each of the divided electrodes. Thereby, the spatial distribution of the irradiated radiation can be detected.
U.S. Pat. No. 6,268,614 discloses another radiation image detector. This radiation image detector is constituted by: a first electrode layer (upper planar electrode) that transmits radiation; a recording photoconductive layer that generates electric charges when irradiated by radiation; a charge transport layer that functions as an insulator with respect to latent image charges and functions as a conductor with respect to charges having a polarity opposite that of the latent image charges; a readout photoconductive layer that generates electric charges when irradiated by readout light; and a second electrode layer (divided electrodes), in which linear electrodes that transmit the readout light are provided, stacked in this order. When recording a radiation image using this radiation image detector, first, radiation which has passed through a subject is irradiated onto the radiation image detector while applying a high negative voltage to the first electrode layer and a high positive voltage to the second electrode layer. The radiation is irradiated onto the recording photoconductive layer, and charge pairs are generated at the portions of the recording photoconductive layer onto which the radiation is irradiated. Positive electric charges among the charge pairs move toward the charged first electrode layer, combine with the negative charges at the first electrode layer, and disappear. Negative electric charges among the charge pairs move toward the charged second electrode layer. However, the charge transport layer functions as an insulator with respect to negative electric charges, as described above. For this reason, the negative electric charges are accumulated at a charge accumulating section at the interface between the recording photoconductive layer and the charge transport layer. The radiation image is recorded by the accumulation of the negative electric charges at the charge accumulating section.
When the recorded radiation image is to be read out from the radiation image detector, readout light is emitted thereon from the side of the second electrode layer. The irradiated readout light enters the readout photoconductive layer, and charge pairs are generated therein. Positive electric charges from among the charge pairs combine with the negative electric charges, which are accumulated at the charge accumulating section. At the same time, negative electric charges from among the charge pairs combine with the positive charges of the charged transparent linear electrodes. The combinations of charges cause current to be detected by current detecting amplifiers connected to the transparent linear electrodes. The current is combined into voltages, which are output as image signals. Residual charges remaining at the charge accumulating section are erased by irradiation of erasing light.
In the radiation image detector disclosed in U.S. Pat. Nos. 6,995,375 and 7,034,312, there are cases in which a portion of signal charges generated due to X ray irradiation is not collected by the divided electrodes, and are accumulated between the divided electrodes. In these cases, problems, such as the occurrence of sensitivity variations and residual images, arise.
Similarly, in the radiation image detector disclosed in U.S. Pat. No. 6,268,614, there are cases in which a portion of signal charges generated due to X ray irradiation is not accumulated in the charge accumulating section, and charge leakage occurs through the charge accumulating section. Charge leakage is likely to occur particularly when high dosage X ray irradiation is performed. The charges that leak through the charge accumulating section within regions above the linear electrodes are discharged by the linear electrodes. On the other hand, the charges that leak through the charge accumulating section within regions above the spaces between the linear electrode are not discharged, and are charged in the spaces between the linear electrodes. This causes a problem that residual images occur.
The problems caused by the charge leakage can be lessened by causing the spaces among adjacent divided electrodes (including transparent linear electrodes) to be narrower. However, there is a possibility that electrical shorts will occur among adjacent divided electrodes if the spaces therebetween are narrowed. Additionally, there is a limit to how narrow the spaces between divided electrodes can be, due to readout noise being increased along with the capacities of the electrodes becoming greater. Accordingly, there is a problem that leaked charges being charged among adjacent divided electrodes cannot be reduced.